Clarify mapping between DELETE and "Ended"

[BenjaminPelletier]

The standard talks a lot about transitioning a flight state to Ended. In the API, this is accomplished by deleting the Operation reference from the DSS. We should make this clearer -- specifically, that "Ended" is not a valid state to instruct the DSS to transition an Operation to, and instead the USS should delete the Operation reference to transition it to Ended.